LEDs are elements that emit light upon passing a forward electric current across a pn junction of a semiconductor, produced using crystals of III-V semiconductors such as GaAs, GaN and the like. In recent years, progress in semiconductor epitaxial growth techniques and light-emitting element processing techniques has resulted in development of LEDs excelling in conversion efficiency, used widely in various fields.
LEDs are composed of a p-type layer and n-type layer formed by epitaxial growth of III-V semiconductor crystals on a monocrystalline growth substrate, and a photoactive layer sandwiched therebetween. Generally, LED light emitting elements are formed by epitaxially growing a III-V semiconductor crystal such as GaN on a growth substrate of monocrystalline sapphire or the like, then forming electrodes and the like (Patent Document 1).
When epitaxially growing III-V semiconductor crystals on a monocrystalline growth substrate, it can be difficult to achieve good single crystal growth due to differences in lattice constant between the monocrystalline growth substrate and the III-V semiconductor crystal. For this reason, a method of forming a buffer layer of GaN or the like at a low temperature on the sapphire substrate, and epitaxially growing GaN thereon has been proposed (Patent Document 2). However, even when this technique is used, the difference in coefficient of linear thermal expansion between the sapphire substrate and the buffer layer of GaN or the like can cause warping of the substrate after epitaxial growth, and in the worst case, the substrate may even crack. Therefore, there is a need for substrate materials having coefficient of linear thermal expansion close to those of III-V semiconductor crystals.
Additionally, monocrystalline growth substrates such as monocrystalline sapphire substrates have the problem of poor thermal conductivity. In the case of monocrystalline sapphire, the thermal conductivity is about 40 W/mK, which is not sufficient to adequately dissipate the heat generated by III-V semiconductor elements of GaN or the like. In particular, the device temperature can rise in high-output LEDs that carry a large current, reducing light emission efficiency and shortening the device lifespan. In response, a method involving epitaxial growth of III-V semiconductor crystals on a monocrystalline growth substrate followed by bonding of a substrate with high thermal conductivity via a metal layer, followed by removal of the monocrystalline growth substrate has been proposed (Patent Document 3). For this reason, materials excelling in thermal conductivity such as copper have been considered as potential high thermal conductivity substrates, but their coefficients of linear thermal expansion differ greatly from those of III-V semiconductor crystals, so they have not proven satisfactory for use with high-output LEDs.    Patent Document 1: JP 2005-117006 A    Patent Document 2: JP H5-73252 B    Patent Document 3: JP 2006-128710 A